This invention relates to temperature effect compensation in semiconductor lasers provided with external cavities. The operation of semiconductor lasers with external cavities has been described by T. Kanada and K. Nawata in `Optical Communictions`, Vol.31 No. 1 pp 81-4 (October 1979) and by D. Renner and J. E. Carrol in `Electronics Letters` Vol.15 No.3 pp 73-4 (February 1979).